Car Light Bulbs and MOT: Which Lights Are Tested and How to Replace Them

A single blown bulb is enough to fail an MOT. It sounds trivial, but lighting faults account for a significant proportion of the outright failures recorded each year across the UK — and the vast majority are entirely preventable with a five-minute walk-round check the evening before your test. This guide covers every lighting item assessed during the MOT, explains what examiners look for with each car light bulb at MOT, and walks you through the practical steps for replacing the most common bulbs yourself.

Which Lights Does the MOT Examiner Check?

The MOT lighting inspection covers a substantial list. Examiners work through each circuit systematically, and a failure on any one item is enough to stop the test in its tracks. The lights assessed include:

• Headlights (main beam and dipped beam): Both must illuminate, be of the correct colour (white or light yellow), and be correctly aimed. Aim is checked on a beam-setting machine — a headlight pointing too high dazzles oncoming drivers and is an automatic failure.
• Front and rear position lights (sidelights): Must be present, illuminated, and of the correct colour. Front must be white or light yellow, rear must be red.
• Brake lights: All brake light units must illuminate when the pedal is depressed, including the high-level third brake light where fitted. A single failed stop light is a failure.
• Rear fog light: Must illuminate and be red. The warning lamp on the dashboard confirming activation is also checked.
• Reversing lights: These are checked for operation (where originally fitted) but a failure here is currently recorded as an advisory rather than an outright failure in most cases.
• Direction indicators (front, rear, side repeaters): All must flash at the correct rate (between 60 and 120 flashes per minute). Rapid flashing caused by a blown bulb causing reduced circuit resistance is a failure.
• Hazard warning lights: All indicators must function simultaneously.
• Number plate light: One of the most commonly overlooked bulbs. The rear number plate must be illuminated when the sidelights are on, and the light must be white — no coloured lenses.

Daytime running lights (DRLs) are inspected on vehicles first used on or after 1 March 2018 — they must illuminate and be of equal brightness on both sides. Cornering lights and adaptive headlight systems are checked for correct operation and warning lamp status. A common MOT failure that surprises many drivers is the dashboard warning lamp: if a lighting system fault lamp remains illuminated during the test, it is recorded as a failure regardless of whether the light itself appears to be working.

Headlight Aim: Why It Matters as Much as the Bulb Itself

Replacing a headlight bulb does not guarantee your headlights will pass the MOT. The aim of each beam is measured precisely using calibrated equipment, and a correctly functioning bulb in a poorly aimed unit is still a failure. The dipped beam must be set so that the cut-off line — the sharp upper edge of the beam pattern — falls within a defined tolerance below the horizontal when the beam setter is positioned at the correct distance from the car.

Headlight aim can shift over time due to worn headlamp levelling motors, damaged adjusters, or simply the vibration of normal driving. If you have recently changed a headlight unit following a collision, had the front bumper removed for repair, or bought the car second-hand, have the aim checked before the MOT. Many garages will set headlight aim as part of a pre-MOT inspection for a modest charge, and it is far cheaper than a retest.

Vehicles fitted with self-levelling or adaptive headlight systems generate a warning lamp when the system detects a fault. Even if the physical beam aim looks correct to the eye, an illuminated headlight fault lamp will cause a failure during the car light bulb MOT test.

How to Replace the Most Common Blown Bulbs Yourself

Many lighting faults are straightforward DIY jobs requiring no specialist tools. The key is to identify the correct bulb type for your vehicle before purchasing a replacement — fitting the wrong base type will either not work or, in some cases, could damage the circuit. Your owner's handbook lists the correct bulb for every position, or you can use an online bulb finder tool by entering your registration.

Number Plate Bulb

Number plate bulbs are almost always accessible without tools on older vehicles. On most cars, the lens cover clips or screws off from the outside of the boot lid or bumper, revealing the bulb holder directly. The standard fitting is a festoon (capless) bulb, usually 5 W or 10 W. Always replace with a white-emitting bulb — blue-tinted aftermarket bulbs can be borderline on colour and are not worth the risk for such a low-cost item. LED festoon replacements are widely available and last significantly longer than standard halogen versions.

Rear Brake and Tail Lights

On the majority of older UK cars, rear light clusters are accessed either from inside the boot (remove the trim panel to reveal the bulb holders) or from outside after removing a few trim screws holding the light cluster. Bayonet-fit bulbs (push and twist to remove) are the most common format. Dual-filament bulbs — where one filament serves the sidelight function and the second serves the stop light — must be replaced with an identical dual-filament part. Fitting a single-filament bulb in a dual-filament socket will result in one function appearing to work while the other fails invisibly.

Halogen Headlight Bulbs

Halogen headlamp bulbs (H1, H4, H7, and H11 are the most common in UK vehicles) are accessible from behind the headlight unit, typically under the bonnet. Remove the wiring connector, release the retaining clip or ring, and the bulb pulls or twists free. Do not touch the glass envelope of a new halogen bulb with bare fingers — skin oils create a hot spot that dramatically shortens bulb life and can cause premature failure. Handle new bulbs with a clean cloth or the protective sleeve supplied.

It is good practice to replace headlight bulbs in pairs. If one has failed, the other is usually at a similar point in its lifespan and a second failure shortly after the MOT is a common and frustrating outcome.

LED and Xenon (HID) Headlights

Factory-fitted LED and HID headlight systems are generally sealed units — individual bulbs cannot be sourced or replaced by the driver. A failed LED strip or HID arc tube typically means the entire headlight module must be replaced, which is a job for a franchised dealer or specialist. Do not attempt to fit aftermarket HID or LED conversion kits into headlight units designed for halogen bulbs: they are not type approved for road use and will almost certainly fail the car light bulb MOT test on both aim and colour grounds.

The Pre-MOT Lighting Check: A Five-Minute Routine

The night before your MOT, carry out this walk-round lighting check:

• Turn on the sidelights and walk around the car to check all four corners.
• Switch to dipped headlights and verify both illuminate with equal brightness. Stand in front of the car to check the cut-off line looks level and symmetrical.
• Switch to main beam and confirm both sides operate.
• Activate the rear fog light — check the red lens illuminates and the dashboard warning lamp lights up.
• Press the brake pedal (use a mirror or ask someone to stand behind the car) and confirm all stop lights work, including the high-level light.
• Cycle through left and right indicators front, rear, and side.
• Activate hazard lights and walk around to confirm all four corners flash.
• Engage reverse gear and check the white reversing light (if fitted) operates.

Any fault found during this check can be fixed at home for a few pounds and a few minutes. Arriving at the MOT station with a known lighting fault is simply paying extra to have the same problem confirmed. Run a MOT check to confirm your test due date, then use a free car check to review any advisory notes from previous tests that flagged lighting issues.

MOT Lighting Failures: What Happens Next

If your car fails the MOT on a lighting fault, the most common outcome is a partial retest. Under current DVSA rules, if the fault is repaired at the same testing station on the same day, only the failed items need to be retested rather than the full inspection. If you take the car away and return on a different day, a full fee applies. This makes it worthwhile to bring spare bulbs for the most common positions to the MOT appointment, so that a bulb failure can be corrected immediately on site.

Check your MOT history to see whether your car has previously been flagged for lighting advisories — a pattern of repeated lighting notes can indicate a deeper fault with a wiring loom or switch assembly that goes beyond simple bulb replacement. Our common MOT failures guide covers lighting faults in the context of the full range of items that catch drivers out each year.